Probing Molecular Packing of Amorphous Pharmaceutical Solids Using X-ray Atomic Pair Distribution Function and Solid-State NMR.

The structural investigation of amorphous pharmaceuticals is of paramount importance in comprehending their physicochemical stability. However, it has remained a relatively underexplored realm primarily due to the limited availability of high-resolution analytical tools. In this study, we utilized the combined power of X-ray pair distribution functions (PDFs) and solid-state nuclear magnetic resonance (ssNMR) techniques to probe the molecular packing of amorphous posaconazole and its amorphous solid dispersion at the molecular level. Leveraging synchrotron X-ray PDF data and employing the empirical potential structure refinement (EPSR) methodology, we unraveled the existence of a rigid conformation and discerned short-range intermolecular C-F contacts within amorphous posaconazole. Encouragingly, our ssNMR 19F-13C distance measurements offered corroborative evidence supporting these findings. Furthermore, employing principal component analysis on the X-ray PDF and ssNMR data sets enabled us to gain invaluable insights into the chemical nature of the intermolecular interactions governing the drug-polymer interplay. These outcomes not only furnish crucial structural insights facilitating the comprehension of the underlying mechanisms governing the physicochemical stability but also underscore the efficacy of synergistically harnessing X-ray PDF and ssNMR techniques, complemented by robust modeling strategies, to achieve a high-resolution exploration of amorphous structures.

Dose Number as a Tool to Guide Lead Optimization for Orally Bioavailable Compounds in Drug Discovery.

Small molecule developability challenges have been well documented over the last two decades. One of these critical developability parameters is aqueous solubility. In general, more soluble compounds have improved oral absorption. While enabling formulation technologies exist to improve bioperformance for low solubility compounds, these are often more complex, expensive, and challenging to scale up. Therefore, to avoid these development issues, medicinal chemists need tools to rapidly profile and improve the physicochemical properties of molecules during discovery. Dose number (Do) is a simple metric to predict whether a compound will be reasonably absorbed based on solubility at an expected clinical dose and represents a valuable parameter to the medicinal chemist defining a clinical candidate. The goal of this mini-Perspective is to present the background of the Do equation and how it can be effectively used to rapidly predict oral absorption potential for molecules in the discovery space.

Flexibility in Drug Product Development: A Perspective.

The process of bringing a drug to market involves innumerable decisions to refine a concept into a final product. The final product goes through extensive research and development to meet the target product profile and to obtain a product that is manufacturable at scale. Historically, this process often feels inflexible and linear, as ideas and development paths are eliminated early on to allow focus on the workstream with the highest probability of success. Carrying multiple options early in development is both time-consuming and resource-intensive. Similarly, changing development pathways after significant investment carries a high "penalty of change" (PoC), which makes pivoting to a new concept late in development inhibitory. Can drug product (DP) development be made more flexible? The authors believe that combining a nonlinear DP development approach, leveraging state-of-the art data sciences, and using emerging process and measurement technologies will offer enhanced flexibility and should become the new normal. Through the use of iterative DP evaluation, "smart" clinical studies, artificial intelligence, novel characterization techniques, automation, and data collection/modeling/interpretation, it should be possible to significantly reduce the PoC during development. In this Perspective, a review of ideas/techniques along with supporting technologies that can be applied at each stage of DP development is shared. It is further discussed how these contribute to an improved and flexible DP development through the acceleration of the iterative build-measure-learn cycle in laboratories and clinical trials.

Understanding molecular mechanisms of biologics drug delivery and stability from NMR spectroscopy.

Protein therapeutics carry inherent limitations of membrane impermeability and structural instability, despite their predominant role in the modern pharmaceutical market. Effective formulations are needed to overcome physiological and physicochemical barriers, respectively, for improving bioavailability and stability. Knowledge of membrane affinity, cellular internalization, encapsulation, and release of drug-loaded carrier vehicles uncover the structural basis for designing and optimizing biopharmaceuticals with enhanced delivery efficiency and therapeutic efficacy. Understanding stabilizing and destabilizing interactions between protein drugs and formulation excipients provide fundamental mechanisms for ensuring the stability and quality of biological products. This article reviews the molecular studies of biologics using solution and solid-state NMR spectroscopy on structural attributes pivotal to drug delivery and stability. In-depth investigation of the structure-function relationship of drug delivery systems based on cell-penetrating peptides, lipid nanoparticles and polymeric colloidal, and biophysical and biochemical stability of peptide, protein, monoclonal antibody, and vaccine, as the integrative efforts on drug product design, will be elaborated.

Molecular packing of pharmaceuticals analyzed with paramagnetic relaxation enhancement and ultrafast magic angle pinning NMR.

Understanding the relationship between the structure and the physicochemical attributes of crystalline pharmaceuticals requires high-resolution molecular details. Solid-state nuclear magnetic resonance (ssNMR) spectroscopy is an indispensable tool for analyzing molecular structures, but often experiences challenges of low spectral resolution and sensitivity, particularly in the characterization of unlabeled pharmaceutical materials. Besides, the relatively long spin-lattice relaxation times in pharmaceutical crystals result in time-consuming data collections. In this study, we utilize ultrafast magic angle spinning (UF-MAS) of the sample at 60 and 110 kHz to enable proton and fluorine spectroscopies for probing the structural details of crystalline posaconazole. Paramagnetic relaxation enhancement (PRE), obtained by doping Cu(ii) ions into the crystalline lattice and coating on particle surface, is implemented to shorten the spin-lattice relaxation time for speeding up the ssNMR acquisition. Our results demonstrate a remarkably improved 1H and 19F resolution and sensitivity, which enables multi-dimensional 1H-1H and heteronuclear 1H-19F correlations. In combination with density functional theory (DFT) calculations of chemical shifts, molecular details of posaconazole are established in terms of 1H and 19F networks for identifying "head-to-tail" and "head-to-head" intermolecular packings, with presumably critical contacts that stabilize the crystalline structure. The PRE and UF-MAS techniques enable the high-resolution structure characterization of fluorinated drug molecules in pharmaceutical formulations at natural abundance.

Atomic-Level Drug Substance and Polymer Interaction in Posaconazole Amorphous Solid Dispersion from Solid-State NMR.

Despite the wide utilization of amorphous solid dispersions (ASDs) for formulating poorly water-soluble drugs, fundamental understanding of the structural basis behind their stability and dissolution behavior is limited. This is largely due to the lack of high-resolution structural tools for investigating multicomponent and amorphous systems in the solid state. In this study, we present what is likely the first publication quantifying the molecular interaction between the drug and polymer in ASDs at an angstrom level by utilizing 19F magic angle spinning (MAS) nuclear magnetic resonance (NMR) techniques. A variant of the 19F-13C rotational-echo and double-resonance (REDOR) technique was developed to quantify interatomic distances by implementing a supercycled symmetry-based recoupling schedule and synchronized simultaneous detection. We successfully deployed the technique to identify "head-to-head" and "head-to-tail" packing of crystalline posaconazole (POSA). To probe molecular interactions between POSA and hypromellose acetate succinate (HPMCAS) in the dispersion, as a major goal of this study, two-dimensional (2D) 1H-19F correlation experiments were performed. The approach facilitated observation of intermolecular hydrogen-to-fluorine contacts between the hydroxyl group of the polymer and the difluorophenyl group of the drug substance. Atomic distance measurement, utilizing the developed 19F-13C REDOR technique, revealed the close proximity of 13COH-19F at 4.3 Å. Numerical modeling analysis suggested a possible hydrogen bonding interaction between the polymer O-H group as an acceptor and POSA fluorine (O-H···F) or difluorophenyl ring (O-H···Ph) as a donor. These 19F MAS NMR techniques, including 2D 19F-1H heteronuclear correlation and 19F-13C atomic distance measurement, may shed light on the nature (i.e., type and strength) of drug-polymer interactions in ASDs and offer a new high-resolution analytical protocol for probing the microstructure of amorphous pharmaceutical materials.

Molecular Mechanism of Crystalline-to-Amorphous Conversion of Pharmaceutical Solids from 19F Magic Angle Spinning NMR.

Crystalline and amorphous materials usually possess distinct physicochemical properties due to major variations in long-range and local molecular packings. Enhanced fundamental knowledge of the molecular details of crystalline-to-amorphous interconversions is necessary to correlate the intermolecular structure to material properties and functions. While crystal structures can be readily obtained by X-ray crystallography, the microstructure of amorphous materials has rarely been explored due to a lack of high-resolution techniques capable of probing local molecular structures. Moreover, there is increasing interest in understanding the molecular nature of amorphous solids in pharmaceutical sciences due to the widespread utilization of amorphous active pharmaceutical ingredients (APIs) in pharmaceutical development for solubility and bioavailability enhancement. In this study, we explore multidimensional 13C and 19F magic angle spinning (MAS) NMR spectroscopy to study the molecular packing of amorphous posaconazole (POSA) in conjunction with the crystalline counterpart. Utilizing methods integrating homonuclear and heteronuclear 1H, 13C, and 19F correlation spectroscopy and atomic 19F-to-13C distance measurements, we identified the major differences in molecular packing between crystalline and amorphous POSA. The intermolecular "head-to-head" interaction along the molecule's major axis, as well as the "head-to-tail" molecular packing perpendicular to the major axis in POSA crystals, was recapitulated by MAS NMR. Furthermore, critical intermolecular distances in the crystal lattice were determined. Most importantly, the head-to-tail contact of two neighboring molecules was found to be preserved in amorphous POSA, suggesting localized molecular order, whereas crucial interactions for head-to-head packing are absent in the amorphous form resulting in long-range disorder. Our study, likely one of the first documented examples, provides molecular-level structural details to understand the molecular mechanism of crystalline-to-amorphous conversion of fluorine-containing drug substances occurring in drug processing and development and establish a high-resolution experimental protocol for investigating amorphous materials.

Mechanism and Impact of Excipient Incompatibility: Cross-Linking of Xanthan Gum in Pediatric Powder-for-Suspension Formulations.

Research on pharmaceutical pediatric powder-for-suspension formulations mainly focuses on chemical and physical stability of the active pharmaceutical ingredient. However, the chemical stability of excipients could also play a key role in governing the quality and performance of the product. The suspending agents that are added into formulations to suspend the active pharmaceutical ingredient particles are critical to ensure the suspension dose accuracy. In this article, we investigate the chemical stability of the suspending agent-xanthan gum-in the presence of other excipients, particularly commonly used acid modifiers (i.e., citric acid, malic acid, succinic acid, and fumaric acid) in pediatric powder-for-suspension formulations. We observed that some of the acid modifiers catalyze cross-linking of xanthan gum during accelerated stability studies in powder blends, which significantly decreases the viscosity of the corresponding constituted suspension, resulting in poor suspendability and dose inaccuracy. Furthermore, we found that the cross-linking of xanthan gum is acid-dependent and that a careful selection of acid modifiers can mitigate the degradation issues of xanthan gum. Finally, we characterized the cross-linked xanthan gum using Fourier transform infrared spectroscopy and solid-state nuclear magnetic resonance and discussed the possible degradation mechanisms.

In-Use Photostability Practice and Regulatory Evaluation for Pharmaceutical Products in an Age of Light-Emitting Diode Light Sources.

This article describes how the increased use of energy-efficient solid-state light sources (e.g., light-emitting diode [LED]-based illumination) in hospitals, pharmacies, and at home can help alleviate concerns of photodegradation for pharmaceuticals. LED light sources, unlike fluorescent ones, do not have spurious spectral contributions <400 nm. Because photostability is primarily evaluated in the International Council of Harmonization Q1B tests with older fluorescent bulb standards (International Organization for Standardization 10977), the amount of photodegradation observed can over-predict what happens in reality, as products are increasingly being stored and used in environments fitted with LED bulbs. Because photodegradation is premised on light absorption by a compound of interest (or a photosensitizer), one can use the overlap between the spectral distribution of a light source and the absorption spectra of a given compound to estimate if photodegradation is a possibility. Based on the absorption spectra of a sample of 150 pharmaceutical compounds in development, only 15% would meet the required overlap to be a candidate to undergo direct photodegradation in the presence of LED lights, against a baseline of 55% of compounds that would, when considering regular fluorescent lights. Biological drug products such as peptides and monoclonal antibodies are also expected to benefit from the use of more efficient solid-state lighting.

Processing Impact on Performance of Solid Dispersions.

The development of a weakly basic compound is often challenging due to changes in pH that the drug experiences throughout the gastrointestinal tract. As the drug transitions from the low pH of the stomach to the higher pH of the small intestine, drug solubility decreases. A stomach with a higher pH, caused by food or achlorhydric conditions brought about by certain medications, decreases even the initial solubility. This decreased drug solubility is reflected in lower in vivo exposures. In many cases, a solubility-enabling approach is needed to counteract the effect of gastrointestinal pH changes. Solid dispersions of amorphous drug in a polymer matrix have been demonstrated to be an effective tool to enhance bioavailability, with the potential to mitigate the food and achlorhydric effects frequently observed with conventional formulations. Because solid dispersions are in a metastable state, they are particularly sensitive to processing routes that may control particle attributes, stability, drug release profile, and bioperformance. A better understanding of the impacts of processing routes on the solid dispersion properties will not only enhance our ability to control the product properties, but also lower development risks. In this study, a weakly basic compound with greatly reduced solubility in higher pHs was incorporated into a solid dispersion via both spray drying and hot melt extrusion. The properties of the solid dispersion via these two processing routes were compared, and the impact on dissolution behavior and in vivo performance of the dispersions was investigated.

New and Evolving Techniques for the Characterization of Peptide Therapeutics.

Advances in technologies related to the design and manufacture of therapeutic peptides have enabled researchers to overcome the biological and technological challenges that have limited their application in the past. As a result, peptides of increasing complexity have become progressively important against a variety of disease targets. Developing peptide drug products brings with it unique scientific challenges consistent with the unique physicochemical properties of peptide molecules. The identification of the proper characterization tools is required in order to develop peptide formulations with the appropriate stability, manufacturability, and bioperformance characteristics. This knowledge supports the build of critical quality attributes and, ultimately, regulatory specifications. The purpose of this review article is to provide an overview of the techniques that are employed for analytical characterization of peptide drug products. The techniques covered are highlighted in the context of peptide drug product understanding and include chemical and biophysical approaches. Emphasis is placed on summarizing the recent literature experience in the field. Finally, the authors provide regulatory perspective on these characterization approaches and discuss some potential areas for further research in the field.

Implications of In-Use Photostability: Proposed Guidance for Photostability Testing and Labeling to Support the Administration of Photosensitive Pharmaceutical Products, Part 3. Oral Drug Products.

The ICH Q1B guidance and additional clarifying manuscripts provide the essential information needed to conduct photostability testing for pharmaceutical drug products in the context of manufacturing, packaging, and storage. As the previous 2 papers in this series highlight for drug products administered by injection (part 1) and drug products administered via topical application (part 2), there remains a paucity of guidance and methodological approaches to conducting photostability testing to ensure effective product administration. Part 3 in the series is presented here to provide a similar approach and commentary for photostability testing for oral drug products. The approach taken, as was done previously, is to examine "worst case" photoexposure scenarios in combination with ICH-defined light sources to derive a set of practical experimental approaches to support the safe and effective administration of photosensitive oral drug products.

Implications of In-Use Photostability: Proposed Guidance for Photostability Testing and Labeling to Support the Administration of Photosensitive Pharmaceutical Products, Part 2: Topical Drug Product.

Although essential guidance to cover the photostability testing of pharmaceuticals for manufacturing and storage is well-established, there continues to be a significant gap in guidance regarding testing to support the effective administration of photosensitive drug products. Continuing from Part 1, (Baertschi SW, Clapham D, Foti C, Jansen PJ, Kristensen S, Reed RA, Templeton AC, Tønnesen HH. 2013. J Pharm Sci 102:3888-3899) where the focus was drug products administered by injection, this commentary proposes guidance for testing topical drug products in order to support administration. As with the previous commentary, the approach taken is to examine "worst case" photoexposure scenarios in comparison with ICH testing conditions to provide practical guidance for the safe and effective administration of photosensitive topical drug products.

Preclinical dose number and its application in understanding drug absorption risk and formulation design for preclinical species.

In the drug discovery setting, the ability to rapidly identify drug absorption risk in preclinical species at high doses from easily measured physical properties is desired. This is due to the large number of molecules being evaluated and their high attrition rate, which make resource-intensive in vitro and in silico evaluation unattractive. High-dose in vivo data from rat, dog, and monkey are analyzed here, using a preclinical dose number (PDo) concept based on the dose number described by Amidon and other authors (Pharm. Res., 1993, 10, 264-270). PDo, as described in this article, is simply calculated as dose (mg/kg) divided by compound solubility in FaSSIF (mg/mL) and approximates the volume of biorelevant media per kilogram of animal that would be needed to fully dissolve the dose. High PDo values were found to be predictive of difficulty in achieving drug exposure (AUC)-dose proportionality in in vivo studies, as could be expected; however, this work analyzes a large data set (>900 data points) and provides quantitative guidance to identify drug absorption risk in preclinical species based on a single solubility measurement commonly carried out in drug discovery. Above the PDo values defined, >50% of all in vivo studies exhibited poor AUC-dose proportionality in rat, dog, and monkey, and these values can be utilized as general guidelines in discovery and early development to rapidly assess risk of solubility-limited absorption for a given compound. A preclinical dose number generated by biorelevant dilutions of formulated compounds (formulated PDo) was also evaluated and defines solubility targets predictive of suitable AUC-dose proportionality in formulation development efforts. Application of these guidelines can serve to efficiently identify compounds in discovery that are likely to present extreme challenges with respect to solubility-limited absorption in preclinical species as well as reduce the testing of poor formulations in vivo, which is a key ethical and resource matter.

Discovery of MK-8970: an acetal carbonate prodrug of raltegravir with enhanced colonic absorption.

Developing new antiretroviral therapies for HIV-1 infection with potential for less frequent dosing represents an important goal within drug discovery. Herein, we present the discovery of ethyl (1-((4-((4-fluorobenzyl)carbamoyl)-1-methyl-2-(2-(5-methyl- 1,3,4-oxadiazole-2-carboxamido)propan-2-yl)-6-oxo-1,6-dihydropyrimidin-5-yl)oxy)ethyl) carbonate (MK-8970), a highly optimized prodrug of raltegravir (Isentress). Raltegravir is a small molecule HIV integrase strand-transfer inhibitor approved for the treatment of HIV infection with twice-daily administration. Two classes of prodrugs were designed to have enhanced colonic absorption, and derivatives were evaluated in pharmacokinetic studies, both in vitro and in vivo in different species, ultimately leading to the identification of MK-8970 as a suitable candidate for development as an HIV therapeutic with the potential to require less frequent administration while maintaining the favorable efficacy, tolerability, and minimal drug-drug interaction profile of raltegravir.

Mechanism of amorphous itraconazole stabilization in polymer solid dispersions: role of molecular mobility.

Physical instability of amorphous solid dispersions can be a major impediment to their widespread use. We characterized the molecular mobility in amorphous solid dispersions of itraconazole (ITZ) with each polyvinylpyrrolidone (PVP) and hydroxypropylmethylcellulose acetate succinate (HPMCAS) with the goal of investigating the correlation between molecular mobility and physical stability. Dielectric spectra showed two mobility modes: α-relaxation at temperatures above the glass transition temperature (Tg) and ß-relaxation in the sub-Tg range. HPMCAS substantially increased the α-relaxation time, with an attendant increase in crystallization onset time and a decrease in crystallization rate constant, demonstrating the correlation between α-relaxation and stability. The inhibitory effect on α-relaxation as well as stability was temperature dependent and diminished as the temperature was increased above Tg. PVP, on the other hand, affected neither the α-relaxation time nor the crystallization onset time, further establishing the link between α-relaxation and crystallization onset in solid dispersions. However, it inhibited the crystallization rate, an effect attributed to factors other than mobility. Interestingly, both of the polymers acted as plasticizers of ß-relaxation, ruling out the latter's involvement in physical stability.

Effect of added alkalizer and surfactant on dissolution and absorption of the potassium salt of a weakly basic poorly water-soluble drug.

Telcagepant potassium salt (MK-0974) is an oral calcitonin gene-related peptide receptor inhibitor investigated for the treatment of acute migraine. Under gastric pH conditions, the salt rapidly gels, then converts to an insoluble neutral form that creates an impervious shell on the tablet surface, resulting in a slow and variable release dissolution rate and poor bioavailability. Early attempts to develop a solid dosage form, including solid dispersion and nanosuspension formulations, resulted in low exposures in preclinical studies. Thus, a liquid-filled soft gelatin capsule (SGC) formulation (oblong 20) was used for clinical studies. However, a solid dosage form was desirable for commercialization. The slow dissolution of the tablet formulations was overcome by using a basifying agent, arginine, and inclusion of a nonionic surfactant, poloxamer 407. The combination of arginine and poloxamer in the formulation created a local transient basic microenvironment that promoted the dissolution of the salt and prevented rapid precipitation of the neutral form on the tablet surface to form the gel layer. The tablet formulation achieved fast absorption and comparable exposure to the SGC formulation. The final optimized 280 mg tablet formulation was successfully demonstrated to be bioequivalent to the 300 mg SGC formulation.